[PATCH] D34204: [LLD][LinkerScript] Allow non-alloc sections to be assigned to segments.

[Rafael Avila de Espindola via llvm-commits]

This sounds odd. What is the use of this feature?

Cheers,
Rafael

Andrew Ng via Phabricator <reviews at reviews.llvm.org> writes:

> andrewng created this revision.
> Herald added a subscriber: emaste.
>
> This patch makes changes to allow sections without the SHF_ALLOC bit to be
> assigned to segments in a linker script.
>
> The assignment of output sections to segments is performed in
> LinkerScript::createPhdrs. Previously, this function would bail as soon as it
> encountered an output section which did not have the SHF_ALLOC bit set, thus
> preventing any output section without SHF_ALLOC from being assigned to a
> segment.
>
> This restriction has now been removed from LinkerScript::createPhdrs and instead
> a check for SHF_ALLOC has been added to LinkerScript::adjustSectionsAfterSorting
> to not propagate program headers to sections without SHF_ALLOC.
>
>
> https://reviews.llvm.org/D34204
>
> Files:
>   ELF/LinkerScript.cpp
>   test/ELF/linkerscript/non-alloc-segment.s
>
>
> Index: test/ELF/linkerscript/non-alloc-segment.s
> ===================================================================
> --- /dev/null
> +++ test/ELF/linkerscript/non-alloc-segment.s
> @@ -0,0 +1,28 @@
> +# REQUIRES: x86
> +# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
> +
> +## Test that non-alloc section .foo can be assigned to a segment.
> +# RUN: echo "PHDRS {text PT_LOAD; foo 0x12345678;} \
> +# RUN:       SECTIONS { \
> +# RUN:           .text : {*(.text .text*)} :text \
> +# RUN:           .foo : {*(.foo)} :foo \
> +# RUN:       }" > %t.script
> +# RUN: ld.lld -o %t --script %t.script %t.o
> +# RUN: llvm-readobj -elf-output-style=GNU -s -l %t | FileCheck %s
> +
> +# CHECK: Program Headers:
> +# CHECK-NEXT:  Type
> +# CHECK-NEXT:  LOAD
> +# CHECK-NEXT:  <unknown>: 0x12345678
> +
> +# CHECK:      Section to Segment mapping:
> +# CHECK-NEXT:  Segment Sections...
> +# CHECK-NEXT:   00     .text
> +# CHECK-NEXT:   01     .foo
> +
> +.global _start
> +_start:
> + nop
> +
> +.section .foo
> + .long 0
> Index: ELF/LinkerScript.cpp
> ===================================================================
> --- ELF/LinkerScript.cpp
> +++ ELF/LinkerScript.cpp
> @@ -753,10 +753,15 @@
>      if (!Cmd)
>        continue;
>  
> -    if (Cmd->Phdrs.empty())
> -      Cmd->Phdrs = DefPhdrs;
> -    else
> +    if (Cmd->Phdrs.empty()) {
> +      OutputSection *Sec = Cmd->Sec;
> +      // Only propagate program headers to sections that are allocated.
> +      if (Sec && (Sec->Flags & SHF_ALLOC))
> +        Cmd->Phdrs = DefPhdrs;
> +    }
> +    else {
>        DefPhdrs = Cmd->Phdrs;
> +    }
>    }
>  
>    removeEmptyCommands();
> @@ -975,8 +980,6 @@
>    // Add output sections to program headers.
>    for (OutputSectionCommand *Cmd : OutputSectionCommands) {
>      OutputSection *Sec = Cmd->Sec;
> -    if (!(Sec->Flags & SHF_ALLOC))
> -      break;
>  
>      // Assign headers specified by linker script
>      for (size_t Id : getPhdrIndices(Sec)) {
>
>
> Index: test/ELF/linkerscript/non-alloc-segment.s
> ===================================================================
> --- /dev/null
> +++ test/ELF/linkerscript/non-alloc-segment.s
> @@ -0,0 +1,28 @@
> +# REQUIRES: x86
> +# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
> +
> +## Test that non-alloc section .foo can be assigned to a segment.
> +# RUN: echo "PHDRS {text PT_LOAD; foo 0x12345678;} \
> +# RUN:       SECTIONS { \
> +# RUN:           .text : {*(.text .text*)} :text \
> +# RUN:           .foo : {*(.foo)} :foo \
> +# RUN:       }" > %t.script
> +# RUN: ld.lld -o %t --script %t.script %t.o
> +# RUN: llvm-readobj -elf-output-style=GNU -s -l %t | FileCheck %s
> +
> +# CHECK: Program Headers:
> +# CHECK-NEXT:  Type
> +# CHECK-NEXT:  LOAD
> +# CHECK-NEXT:  <unknown>: 0x12345678
> +
> +# CHECK:      Section to Segment mapping:
> +# CHECK-NEXT:  Segment Sections...
> +# CHECK-NEXT:   00     .text
> +# CHECK-NEXT:   01     .foo
> +
> +.global _start
> +_start:
> + nop
> +
> +.section .foo
> + .long 0
> Index: ELF/LinkerScript.cpp
> ===================================================================
> --- ELF/LinkerScript.cpp
> +++ ELF/LinkerScript.cpp
> @@ -753,10 +753,15 @@
>      if (!Cmd)
>        continue;
>  
> -    if (Cmd->Phdrs.empty())
> -      Cmd->Phdrs = DefPhdrs;
> -    else
> +    if (Cmd->Phdrs.empty()) {
> +      OutputSection *Sec = Cmd->Sec;
> +      // Only propagate program headers to sections that are allocated.
> +      if (Sec && (Sec->Flags & SHF_ALLOC))
> +        Cmd->Phdrs = DefPhdrs;
> +    }
> +    else {
>        DefPhdrs = Cmd->Phdrs;
> +    }
>    }
>  
>    removeEmptyCommands();
> @@ -975,8 +980,6 @@
>    // Add output sections to program headers.
>    for (OutputSectionCommand *Cmd : OutputSectionCommands) {
>      OutputSection *Sec = Cmd->Sec;
> -    if (!(Sec->Flags & SHF_ALLOC))
> -      break;
>  
>      // Assign headers specified by linker script
>      for (size_t Id : getPhdrIndices(Sec)) {